Devices and methods for backscatter communication using one or more wireless communication protocols including Bluetooth low energy examples

ABSTRACT

Examples described herein include devices and methods that may facilitate interoperability between backscatter devices and wireless communication devices. For example, backscatter devices and methods for backscattering are described that provide a transmitted backscattered signal formatted in accordance with a wireless communication protocol (e.g. Bluetooth Low Energy, WiFi, IEEE 802.11, or IEEE 802.15.4). Such communication may reduce or eliminate any modifications required to wireless communication devices necessary to receive and decode backscattered signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of patent application Ser. No.16/297,355 filed Mar. 8, 2019, issued as U.S. Pat. No. 10,693,521 onJun. 23, 2020, which is a continuation of patent application Ser. No.16/119,055 filed Aug. 31, 2018, abandoned, which is a continuation ofpatent application Ser. No. 15/249,167, filed Aug. 26, 2016, issued asU.S. Pat. No. 10,079,616 on Sep. 18, 2018, which is acontinuation-in-part of International Application PCT/US2015/066820,filed Dec. 18, 2015 (the '820 application). The instant application alsoclaims the benefit under 35 U.S.C. 119 of the earlier-filed provisionalapplication 62/210,900, filed Aug. 27, 2015. The '820 application claimsthe benefit under 35 U.S.C. 119 of earlier-filed provisionalapplications 62/094,277, filed Dec. 19, 2014 and 62/107,149 filed Jan.23, 2015. These applications and issued patents are hereby incorporatedby reference in their entirety for any purpose.

TECHNICAL FIELD

Examples described herein are directed generally to wireless datatransmission. In particular, examples are described that transmit datawirelessly by backscattering a signal such that the backscattered signalis compatible with a wireless communication protocol utilized by areceiving device.

BACKGROUND

Wireless communication devices generally transmit information bygenerating a radiofrequency carrier using a circuit such as anoscillator, and modulating information onto the carrier wave usingamplitude modulation, frequency modulation, phase modulation, quadratureamplitude modulation (QAM) or other techniques including a combinationof the aforementioned modulation types. Multiple such modulated signalsmay be combined to form more complex schemes such as orthogonalfrequency division multiplexing (OFDM). The carrier is usually asinusoidal voltage at a radio frequency; that is a frequency at whichenergy may be propagated in the form of an electromagnetic wave byconnecting the sinusoidal voltage to an antenna. The modulation processmodifies the amplitude, frequency, and/or phase of the carrier in a timevarying manner to convey information. Examples of conventional wirelesscommunication devices include analog communication systems such asanalog AM and FM broadcast radio as well as digital communicationsystems such as the widely used Wi-Fi (e.g. IEEE 802.11) and Bluetoothdata communication standards as well as digital television (e.g. DTV)and digital broadcast radio standards.

Generally, conventional wireless communication devices haveradiofrequency carrier generation and the modulation processes carriedout in a single device or installation of interconnected devices.

In contrast, backscatter devices generally refer to an alternativecommunication method where carrier generation and modulation areperformed in separate devices. For example, a carrier frequency may begenerated in a first device that emits an electromagnetic carrier wave.A second device carries out the modulation process by scattering orreflecting the carrier wave, thus affecting the amplitude, frequency,and/or phase of the carrier emitted by the first device. This can beachieved by modulated scattering; that is by selective reflection of theincident carrier wave by means of a modulator circuit. Backscatterdevices, requiring a modulator which may be a simple as a transistor,may be quite simple and low power.

Backscatter communication is widely used in ultra-high frequency RFIDsystems. By using modulated backscatter to communicate, RFID tags arepower efficient compared to alternative approaches using conventionalwireless communication schemes. However, RFID tags require a specializedreader or receiver hardware to receive the backscattered signal. RFIDreaders, for example, are complex devices which include a transmittercircuit, which performs the carrier wave generation process, along witha receiver circuit, which receives the modulated backscatter signal andextracts the data transmitted by the RFID tag. This specialized hardwarepresents a cost and complexity burden to users of the RFID system, inthat RFID readers must be purchased, installed, and maintained on a datacommunication network to take advantage of the RFID tags.

SUMMARY

Example devices are described herein. An example device may include anantenna configured to receive an incident signal having a carrierfrequency. The device may further include a modulator and a symbolgenerator. The symbol generator may be configured to provide asubcarrier frequency. The symbol generator may further be configured tocontrol the modulator to backscatter the incident signal having thecarrier frequency using the subcarrier frequency to provide abackscattered signal to the antenna. The backscattered signal mayinclude a bandpass signal in a predetermined frequency range.

In some examples, the predetermined frequency range is a range specifiedby a wireless communication standard.

In some examples, the predetermined frequency range is a range of anadvertising channel specified by a Bluetooth Low Energy specification.

In some examples, the symbol generator may be configured to provide thebackscattered signal in part by mixing the subcarrier frequency with thecarrier frequency.

In some examples, the symbol generator may be configured to provide thebackscattered signal in part by mixing a harmonic of the subcarrierfrequency with the carrier frequency.

In some examples, the modulator may include a field effect transistor.

In some examples, the backscattered signal may include a packet. In someexamples, the packet may include a preamble, an access address, apayload data unit, and a cyclic redundancy check.

In some examples, the device may further include a frequency sourcecoupled to the symbol generator. The frequency source may be configuredto provide the subcarrier frequency. In some examples, the device mayinclude multiple frequency sources coupled to the symbol generator. Thesymbol generator may be configured to select at least one of themultiple frequency sources for use in providing the backscatteredsignal. The symbol generator may be configured to select at least one ofthe multiple frequency sources in accordance with data provided to thesymbol generator. In some examples, at least one of the multiplefrequency sources is modulated in amplitude, frequency, and/or phase.

In some examples, the subcarrier frequency may be modulated inamplitude, frequency, and/or phase.

In some examples, the backscattered signal may be an orthogonalfrequency division multiplex (OFDM) signal.

In some examples, the incident signal may include a data-carryingsignal. In some examples, the incident signal may include a signalarranged in accordance with a wireless communication protocol. In someexamples, the incident signal may include a Bluetooth signal and in someexamples the backscattered signal may include a Bluetooth advertisingpacket. In other examples, the incident signal may include a WiFisignal. In further examples, the backscattered signal may include a WiFisignal such as a beacon frame. In some examples, the incident signal mayinclude a Zigbee or IEEE 802.15.4 signal. In some examples, thebackscattered signal may include a Zigbee or IEEE 802.15.4 beacon frame.

Examples of methods are described herein. An example method may includereceiving an incident signal having a carrier frequency. The method mayinclude backscattering the incident signal to provide a backscatteredsignal. The backscattering may include modulating, using a backscatterdevice, impedance presented to at least one antenna in accordance withdata to be provided in the backscattered signal, and mixing the carrierfrequency with at least one subcarrier provided by the backscatterdevice.

In some examples, the mixing may result in a bandpass signal having apredetermined frequency range. In some examples, the predeterminedfrequency range may include a range of a channel in accordance with awireless communication standard. In some examples, the wirelesscommunication standard comprises Bluetooth Low Energy.

In some examples, modulating include modulating the amplitude,frequency, and/or phase of the backscattered signal in a patternindicative of the data to be provided in the backscattered signal.

In some examples, the data to be provided in the backscattered signalincludes a packet having a preamble, an access address, a payload dataunit, and a cyclic redundancy check.

In some examples, a method further includes transmitting thebackscattered signal.

In some examples, the backscattered signal includes a reading of asensor associated with a device providing the backscattered signal.

In some examples, the backscattered signal may include an identificationof an asset associated with a device providing the backscattered signal.

In some examples, the device providing the backscattered signal includesa tag.

In some examples, the incident signal may include a data-carryingsignal. In some examples, the incident signal may include a signalarranged in accordance with a wireless communication protocol. In someexamples, the incident signal may include a Bluetooth signal and in someexamples the backscattered signal may include a Bluetooth advertisingpacket. In other examples, the incident signal may include a WiFisignal. In further examples, the backscattered signal may include a WiFisignal such as a beacon frame. In some examples, the incident signal mayinclude a Zigbee or IEEE 802.15.4 signal. In further examples, thebackscattered signal may include a Zigbee or IEEE 802.15.4 beacon frame.

Examples of systems are described herein. An example system may includea signal source configured to provide an incident signal, a backscatterdevice configured to provide a backscattered signal, and a wirelesscommunication device configured to receive the backscattered signal. Thebackscatter device may include an antenna configured to receive theincident signal having a carrier frequency, a modulator, and a symbolgenerator. The symbol generator may be configured to provide asubcarrier frequency, and the symbol generator may be further configuredto control the modulator to backscatter the incident signal having thecarrier frequency using the subcarrier frequency to provide abackscattered signal, the backscattered signal including a bandpasssignal in a predetermined frequency range and/or channel. The wirelesscommunication device may be configured to receive the backscatteredsignal using components also used to receive communication signals whichare not backscattered.

In some examples, the wireless communication device and the signalsource are wholly or partially integrated into a same device.

In some examples, the same device is configured to operate in a fullduplex mode for transmission of the incident signal in one channel andreceipt of the backscattered signal in a second channel different fromthe one channel.

In some examples, the wireless communication device and the signalsource are separate devices.

In some examples, the backscatter device may include a receiver.

In some examples, the incident signal has an incident signal durationand a duration of the backscattered signal may be less than the incidentsignal duration such that the backscattered signal is provided by thebackscatter device during a time the incident signal is present. In someexamples, the presence of the incident signal may be detected by areceiver of the backscatter device to determine a time to provide thebackscatter signal (e.g. while the incident signal is present). In someexamples, the backscatter signal may be provided at a time indicated bya deterministic or a randomized timer.

In some examples, the incident signal and the backscattered signal areeach formatted in accordance with a wireless communication protocol. Insome examples, the incident signal and the backscatter signal have thesame wireless communication protocol. In further examples, the incidentsignal and the backscatter signal have different wireless communicationprotocols.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a system including a backscatterdevice in accordance with examples described herein;

FIG. 2 is a schematic illustration of a backscatter device in accordancewith examples described herein;

FIG. 3 is a flowchart illustrating a method in accordance with examplesdescribed herein; and

FIG. 4 is a schematic illustration of an example packet compatible withthe BTLE specification.

FIG. 5 is a schematic illustration of a backscatter device arranged inaccordance with examples described herein.

FIGS. 6A-6C are schematic illustrations of spectra of a Bluetooth sourcesignal in Channel 38 (FIG. 6A) and the backscattered ‘0’ signal inChannel 37 (FIG. 6B) and the backscattered ‘1’ signal in Channel 37(FIG. 6C).

DETAILED DESCRIPTION

Certain details are set forth below to provide a sufficientunderstanding of embodiments of the disclosure. However, it will beclear to one skilled in the art that embodiments of the disclosure maybe practiced without various of these particular details. In someinstances, well-known device components, circuits, control signals,timing protocols, and software operations have not been shown in detailin order to avoid unnecessarily obscuring the described embodiments ofthe disclosure.

Examples described herein include backscatter devices (e.g. transmittersor transceivers) that utilize backscattered signals to communicate witheach other and/or other devices in accordance with established wirelesscommunication protocols. For example, a system may include a backscatterdevice that is configured to transmit data by modulating a backscatteredversion of an incident signal and mixing the carrier frequency of theincident signal with a subcarrier frequency such that a resultingbackscatter signal includes a bandpass signal having a predeterminedfrequency range. The predetermined frequency range may, for example, bea frequency range specified by a wireless communication protocol, suchas Bluetooth Low Energy (BLE), sometimes called Bluetooth Smart. Otherwireless communication protocols such as WiFi (IEEE 802.11), Zigbee,IEEE 802.15.4, etc. may also be used. Examples described herein mayaccordingly include systems, devices and methods for providingbackscatter signals which may have the same characteristics asconventional wireless communication signals, allowing conventionalwireless devices to receive backscattered signals instead of restrictingbackscatter communications to specialized readers in some examples.Accordingly, wireless communication devices may receive examples ofbackscattered signals described herein using the same components (e.g.chipsets, other hardware, software, or combinations thereof) used toreceive communication signals which may not be backscattered signals.

FIG. 1 is a schematic block diagram of a system including a backscatterdevice in accordance with examples described herein. The system mayinclude a signal source 100, which may be configured to provide a signal130 using antenna 105. The system may include a backscatter device 110which may be configured to receive the signal 130 using the antenna 115and modulate a backscattered version of the signal 130 to provide atransmitted backscatter signal 135 (e.g. backscattered signal) using theantenna 115. The system may further include a wireless communicationdevice 120 that may receive the transmitted backscatter signal 135 usingan antenna 125. The transmitted backscatter signal 135 may beconstructed in accordance with established wireless communicationprotocols, such that the wireless communication device 120 may receiveand decode the transmitted backscatter signal 135 without a need forcustom programming (e.g., firmware, software) or hardware specific tocommunication with the backscatter device 110.

The signal source 100 may generally be any device that is capable oftransmitting a suitable signal 130 for backscatter by the backscatterdevice 110. Generally, the signal 130 may be a radio frequency signal,such as a wireless communication signal. The signal 130 may have acarrier frequency (e.g. a frequency of a carrier wave that may bemodulated with an input signal to provide data in the signal 130). Thesignal 130 may generally be implemented using any signals which may bereceived and backscattered by backscatter devices described herein. Thesignal 130 may be implemented using an RF signal including a wirelesscommunication signal.

Examples of signals used to implement the signal 130 include, but arenot limited to, television transmission signals, radio transmissionsignals, cellular communication signals, Bluetooth signals, Wi-Fi (e.g.IEEE 802.11), Zigbee, and IEEE 802.15.4 signals. Devices which may beused to implement the signal source 100 include but are not limited totelevision transmitters, base stations including cellular base stations,AM or FM broadcast stations, digital radio stations, radar, Wi-Fi (e.g.IEEE 802.11) access points, Bluetooth devices, mobile devices,telephones (including cellular telephones), computers, routers,appliances, transceivers, tablets, and watches. In some examples thesignal source 100 may be terrestrial while in other examples the signalsource 100 may be located on an aircraft, satellite or spacecraft. Itshould be understood that any externally (e.g. external to thebackscatter device 110) generated carrier having at least one frequencycomponent in the frequency range of interest (sometimes referred to asF_(carrier)) may be employed. In some examples, the signal source 100may supply at least a portion of the operating power for the backscatterdevice 110. In some examples, backscatter device 110 may include an RFenergy harvesting circuit to extract all or portions of its operatingpower from the signal 130 (and/or other environmental signals).

The signal 130 may be present in the environment from signal sourcesalready present in an environment, and/or the signal 130 may be providedby a signal source placed in an environment for the purpose of providinga signal to the backscatter device 110. While shown as having oneantenna 105 the signal source 100 may be implemented having any numberof antennas, including a phased array antenna, or amultiple-input-multiple-output (M IMO) array of antennas. In someexamples, the signal 130 may itself be a data-carrying signal which mayitself be arranged in accordance with a wireless communication protocol(e.g. a Bluetooth signal and/or Bluetooth Low Energy (BLE) signal, or aWiFi 802.11 signal).

The signal source 100 may include a frequency source, such as anoscillator or frequency synthesizer, which may supply radio frequencyenergy to the antenna 105, in some examples via a power amplifierincluded in the signal source 100. The frequency source may include oneor more of a fixed frequency source, a frequency hopping source, or adirect sequence spread spectrum source. It may be powered by batteries,by an AC power source, or by energy harvested from its environment (suchas via a solar cell or a thermal or vibrational energy harvester). Thesignal source 100 (e.g. a transmitter) may be fixed in location or itmay be mobile, as in a handheld or vehicle mounted application.

In some examples the signal source 100 may include and/or be co-locatedwith a receiver connected to the same antenna 105 or antenna array. Insome examples the signal source 100 may be implemented using an RFIDreader.

The backscatter device 110 may be implemented, for example, using a tag.In some examples, the backscatter device 110 may be implemented using adevice for which low power communication is desirable, such as a tag,sensor node, or the like. Tags implementing the backscatter device 110may be associated with (e.g. placed on and/or proximate to) any of avariety of items to provide information about the items. Such itemsinclude, but are not limited to, appliances, food storage containers,inventory items such as personal electronics, and portions of abuilding. While shown as having one antenna 115, the backscatter device110 may utilize any number of antennas in some examples.

The backscatter device 110 may modulate a backscattered version of thesignal 130 from the signal source 100 to provide a transmittedbackscatter signal 135 encoded with data to the wireless communicationdevice 120. The transmitted backscatter signal 135 may be formatted inaccordance with predetermined wireless communication standards, such asbut not limited to the Bluetooth Low Energy (also called BluetoothSmart) standard. There are many different wireless communicationstandards, each of which may have a specified frequency plan, modulationscheme, and packet data format, among other specified parameters. Aconventional wireless communication standard may be used in someexamples, at least owing to the ease with which the backscattered signalmay be received and decoded by existing devices. For example, BLEdevices may be widely deployed in smart phones, tablets, PCs, and otherdevices from major manufacturers such as Apple and Samsung. Thesecompanies have adopted the Bluetooth 4.0 standard including the BLE modeof operation which was generally created to accommodate low energyapplications. One driver for this technology has been the demand forbeacons, such as the Apple iBeacon, which may provide location awarenessto iOS devices. In some examples, several BLE features may be leveragedin examples described herein. Sensor ID and data may be transferred inbroadcast “advertising packets”, without requiring acknowledgements.Also, the three advertising channels defined in the BLE spec use a fixedmodulation scheme (Gaussian-shaped binary FSK at 1 Mbps), in three fixedfrequency channels centered on 2402 MHz, 2426 MHz, and 2480 MHz. Also,every BLE receiver listens for incoming advertising packets across allthree advertising channels, so reception of advertising packets on anyone channel is sufficient for the message to be received. These featuresof BLE may be leveraged by systems, devices, and methods describedherein to provide backscattered communication. In other examples, thebeacon frames of wireless communication standards such as WiFi, IEEE802.11, Zigbee, IEEE 802.15.4, or other communication standards may beused analogously to the “advertising packets” of the BLE spec.

Data encoded in the transmitted backscatter signal 130 by thebackscatter device 100 may, for example, be related to data receivedfrom a sensor or an input, or may be related to an identity or parameterof an item with which the backscatter device 110 is associated (e.g.temperature in a portion of a building, identity of an inventory item,temperature of a food storage container, a biological or physiologicalsignal including measurement of a parameter relevant to human or animalhealth such as heart rate, blood pressure, body chemistry such as oxygenlevel, glucose level, the level of another analyte, or neural data suchneural recording data or muscle activity such as electromyelogram or EMGdata).

Backscatter communication generally includes modulating the reflectionof an incident signal at an antenna, rather than generating the signalitself. The signal 130 used by the backscatter device 110 may include asignal having a carrier frequency that is provided by the signal source100 for another purpose, such as a television broadcast or cellularcommunication between a base station and a mobile device, ortransmission between an access point and a mobile device, ortransmissions between two mobile devices using one or more of theaforementioned wireless communication protocols. In some examples, thetransmitted backscatter signal 135 may be encoded with data using amodulation scheme. To generate the backscattered signal, the backscatterdevice 110 may modulate the impedance of one or more antennas, such asthe antenna 115, to alternate between two or more discrete states, e.g.,including in some embodiments reflecting and not-reflecting. Thereflecting state of the antenna 115 may provide a reflection of thesignal 130, and the non-reflecting state may not reflect the signal 130.Thus, the backscatter device 110 may indicate either a ‘0’ or a ‘1’ bitby switching the state of the antenna 115 between the reflecting andnon-reflecting states.

Switching the state of the antenna 115 of the backscatter device 110 mayinclude adjusting an impedance of a load attached to the terminals ofthe antenna 115. The magnitude and/or phase of the scattered signal fromthe antenna 115 is typically determined by the difference in theimpedance values of the load attached to the terminals of the antenna115. By modulating the electrical impedance presented to the antenna115, the magnitude and/or phase of incident energy that is scattered ismodulated, thus enabling information to be transmitted. For example, ina first state, the antenna 115 may have a first impedance (e.g., a shortcircuit) to a reference node and may reflect the signal 130 to provide atransmitted backscatter signal 135 that has a first signal magnitude andphase. In a second state, the antenna 115 may have a second impedance(e.g., an open circuit) to the reference node, and may reflect thesignal 130 to provide a backscatter signal 135 that has a second signalmagnitude and phase. The first magnitude may be greater or less than thesecond magnitude. This yields an amplitude shift keying (ASK)backscattered signal in some examples. In some examples, thebackscattered signal may differ primarily in phase between the firststate and the second state. This yields a phase shift keying (PSK)backscattered signal. It should be understood that more than twomagnitude states may be employed, thus yielding a pulse amplitudemodulated (PAM) backscattered signal. It should further be understoodthat more than two phase states, such as M states, may be employed, thusyielding an M-ary PSK backscattered signal. In some examples, theimpedances of the loads attached to the terminals of the antenna arechosen to affect both the magnitude and the phase of the backscatteredsignals in each of several states. In such embodiments, a quadratureamplitude modulation (QAM) backscattered signal may be produced.

By opening and closing the modulating switch in a time varying pattern,the scattering or reflectivity will be time varying, and thusinformation may be conveyed by the scattered or reflected signal. Insome embodiments, the modulating switch is opened and closed once foreach transmitted symbol. The rate of this time varying pattern may thenbe referred to as the symbol rate of the backscattered signal. Thesymbol rate is the rate at which the modulator changes its impedancestate to convey different pieces of information (e.g. groups of one ormore bits). It should be understood that circuits or structures otherthan a switch may be used to change the impedance state of the loadconnected to the antenna 115. Such devices as a PIN diode, a varactordiode, a field effect transistor, a bipolar transistor, or circuitcombinations of these elements may also be used to change the impedancestate of the load connected to antenna 115.

The backscatter device 110 may include a modulator that may function tomodulate the backscatter of the signal 130, e.g. to switch an impedanceof the load attached to antenna 115 from a non-reflecting to areflecting state. The backscatter device 110 may also provide asubcarrier frequency. In some examples, the subcarrier frequency may beprovided, for example, by an oscillator. The switching or modulatingaction of the backscatter device 110 may mix the subcarrier frequencywith the carrier frequency of the signal 130 to adjust a frequencycomponent of the transmitted backscatter signal 135. In this manner, thetransmitted backscatter signal 135 may include a bandpass signalcomponent having a predetermined frequency range, for example afrequency range specified by a wireless communication standard.

Examples of backscatter devices described herein, including thebackscatter device 110 of FIG. 1, may have parameters selected toproduce frequency components corresponding to at least one band-passsignal in the frequency spectrum of the scattered or reflected signal.These frequency components may be select to be compatible with aband-pass signal expected by a wireless communication device (e.g. thewireless communication device 120 of FIG. 1) such that the wirelesscommunication device will accept and properly decode the transmittedbackscattered signal. The transmitted backscattered signal may containother frequency components that are outside of the desired band-passsignal but these components may be out-of-band with respect to thecommunication signal and thus discarded by the wireless communicationdevice 120.

In some examples, the backscatter device 110 may include a receiver 150.The receiver 150 may be used to detect a presence of the signal 130. Insome examples, the receiver 150 may detect energy related to thepresence of the signal 130. In some examples, the receiver 150 maydecode all or a portion of the signal 130. For example, the receiver 150may obtain an expected duration of the signal 130 by decoding at least aportion of the incident signal. The receiver 150 may be utilized by thebackscatter device 110 to determine when to provide the backscattersignal 135. For example, the backscatter device 110 may provide thebackscatter signal 135 during a time the signal 130 is incident on thebackscatter device 110. Accordingly, in some examples, the backscatterdevice 110 may select a time at which to begin backscattering based on asignal from the receiver 150 indicative of signal 130 being incident onthe backscatter device 110. In some examples, backscatter device 110 mayselect a time at which to stop backscattering based on a signal from thereceiver 150 indicative of signal 130 being absent and/or signal 130being a predetermined time away from ending (e.g. when a markerindicative of an end of the signal 130 is received by the receiver 150).

The wireless communication device 120 may receive the transmittedbackscatter signal 135 at the antenna 125. While one antenna 125 isshown, multiple antennas may also be used. The wireless communicationdevice 120 may be implemented using any device capable of wirelesscommunication, including but not limited to, a cellular telephone,computer, server, router, laptop, tablet, wearable device, watch,appliance, automobile, or airplane. The wireless communication device120 may be configured to (e.g. include hardware and/or firmware andsoftware for) communicate using a particular protocol for a wirelesscommunication signal (e.g. Bluetooth Low Energy, Bluetooth Smart, Wi-Fi,CDMA, TDMA). The backscatter device 110 may provide a transmittedbackscatter signal 135 formatted in accordance with the wirelesscommunication protocol expected by the wireless communication device120. For example, the backscatter signal 135 may be a Bluetooth signal(e.g. such as an advertising packet), a Wi-Fi signal (e.g. such as abeacon frame), and/or a ZigBee signal. For example, the backscattersignal 135 may be a IEEE 802.15.4 beacon frame. In this manner, nofurther software, firmware, or hardware may be required for the wirelesscommunication device 120 to receive and decode the transmittedbackscatter signal 135 than is required for the wireless communicationdevice 120 to receive and decode received signals from other sourcesthat are formatted in accordance with the wireless communicationprotocol.

The wireless communication device 120 may employ a frequency shiftkeying (FSK) or Gaussian frequency shift keying (GFSK) standard havingat least one or more specified frequency deviations, one or morespecified channel center frequencies, and one or more specified symbolrates. In some examples, the aforementioned FSK or GFSK standard is thatof the Bluetooth Low Energy specification as defined by the BluetoothSpecial Interest Group (SIG). Accordingly, in some examples thebackscatter device 110 may provide a transmitted backscatter signal 135compatible with the FSK or GFSK standard employed by the wirelesscommunication device 120. In some examples, features of the BLEspecification (e.g. the use of broadcast packets on advertisingchannels) may be used by systems described herein such that examplebackscatter devices may provide backscattered signals that can bereceived by unmodified BLE devices. From the point of view of the BLEreceivers the backscattered signals may be indistinguishable fromconventional BLE transmissions. The backscatter devices may use either acontinuous wave signal or a data-carrying signal (e.g. a BLE signal) inthe environment as a carrier signal to generate a binary FSKbackscattered signal. In some examples, the backscattered signal mayhave a data rate of 1 Mbps and may be received as a BLE advertisingpacket. In some examples, a dateless signal in the (e.g. a CW source)and an information carrying signal in the form of a BLE messages may bemodified to contain a BLE advertising packet specified by thebackscatter device. The mixing products produced by backscattertechniques described herein may allow for fundamental mode and harmonicmode creation of BLE messages.

The wireless communication device 120 may employ a phase shift keying(PSK) standard. Accordingly, in some examples the backscatter device 110may provide a transmitted backscatter signal 135 compatible with the PSKstandard. It should be appreciated that the PSK signal so generated mayuse two distinct phases to encode a symbol or a bit, or it mayalternatively have more than two distinct phases to encode a symbol or agroup of bits as in M-ary PSK.

The wireless communication device 120 may employ an amplitude shiftkeying (ASK) standard. Accordingly, in some examples the backscatterdevice 110 may provide a transmitted backscatter signal 135 compatiblewith the ASK standard. It should be appreciated that the ASK signal sogenerated may use two distinct amplitudes to encode a symbol or a bit,or it may alternatively have more than two distinct amplitudes to encodea symbol or a group of bits as in pulse amplitude modulation (PAM).

The wireless communication device 120 may employ a quadrature amplitudemodulation (QAM) standard. Accordingly, in some examples the backscatterdevice 110 may provide a transmitted backscatter signal 135 compatiblewith the QAM standard. It should be appreciated that the QAM signal mayhave more than two distinct amplitudes and phase combinations to encodea symbol or a group of bits, as in M-ary QAM.

The wireless communication device 120 may employ an orthogonal frequencydivision multiplexing (OFDM) standard and/or technique. Accordingly, insome examples the backscatter device 110 may provide a transmittedbackscatter signal 135 compatible with the OFDM standard and/ortechnique. This may be achieved by modulating the backscatter signal 135with more than one subcarrier frequency at the same time. Eachsubcarrier may in turn be modulated with ASK, PAM, PSK, or QAM to formthe OFDM backscattered signal.

In some examples, the wireless communication device 120 and the signalsource 100 may be separate devices (as shown in FIG. 1). In someexamples, the wireless communication device 120 (e.g. the device thatmay receive the backscattered signal) and the signal source 100 (e.g.the device that may provide an incident signal for backscattering) maybe wholly or partially integrated into a same device. For example, adevice may be used including circuitry having a full duplex mode fortransmission in one channel (e.g. a channel in which a signal source maytransmit a signal for backscattering by the backscatter device 110) andreceiving in another channel (e.g. a channel in which a backscatteredsignal is provided from the backscatter device 110).

In some examples, the signal source 100 may provide a wirelesscommunication signal formatted in accordance with a wirelesscommunication protocol (e.g. a Bluetooth and/or BLE signal, a WiFisignal, a ZigBee signal, or combinations thereof). The wirelesscommunication device 120 may receive both the wireless communicationsignal from the signal source 100 and the backscattered signal from thebackscatter device 110.

While FIG. 1 depicts one backscatter device 110, the system may includemore than one backscatter device, and multiple backscatter devices maybe in communication with the wireless communication device 120 usingsignals backscattered from the signal source 100. Moreover, while FIG. 1depicts one signal source 100, in some examples, the system may includemore than one signal source.

In some examples, multiple backscatter devices may simultaneously (e.g.wholly and/or partially simultaneously) backscatter the signal 130 froma signal source 100 to form multiple backscatter signals in multiplechannels corresponding to the channels of a single wirelesscommunication protocol or standard. In some examples, multiplebackscatter devices may simultaneously (e.g. wholly and/or partiallysimultaneously) backscatter the signal 130 from a signal source 100 toform multiple backscatter signals in multiple channels corresponding tothe channels of multiple wireless communication protocols or standards.

In some examples, multiple backscatter devices may sequentiallybackscatter the signal 130 from a signal source 100 to form multiplebackscatter signals occupying multiple channels at different times. Insome examples, a single backscatter device may employ its symbolgenerator (e.g. symbol generator 230 of FIG. 2) to generate multiplesimultaneous backscatter signals in multiple channels at the same time.

In some examples, communication between a signal source 100 and awireless communication device 120 may be conducted simultaneously (e.g.wholly and/or partially simultaneously) with a backscatter signalgenerated by the backscatter device 110. The backscatter signal 135generated by the backscatter device 110 may be received either by thedepicted wireless communication device 120 or by another wirelesscommunication device implementing either the same or a differentwireless communication standard as that used by signal source 100.

FIG. 2 is a schematic illustration of a backscatter device in accordancewith examples described herein. The backscatter device 200 may be used,for example to implement the backscatter device 110 of FIG. 1. Thebackscatter device 200 includes an antenna 215, a modulator 220, and asymbol generator 230. The modulator 220 may modulate an impedance of theantenna 215 to change the magnitude and/or phase of an incident signal,e.g. the signal 130 of FIG. 1.

The antenna 215 may be used to implement the antenna 115 of FIG. 1 insome examples. The antenna 215, during operation, may receive anincident signal having a carrier frequency, such as the signal 130 ofFIG. 1. The antenna 215 may further transmit a transmitted backscatteredsignal, e.g. the backscatter signal 135 of FIG. 1, by reflecting and/orabsorbing portions of the signal 130 as controlled by the modulator 220and symbol generator 230. The reflected and/or absorbed portions of thesignal 130 may be modulated in combinations of amplitude and phase, andsubcarrier frequency and phase as described herein, for example.

The modulator 220 may generally be implemented using any device capableof modulating an impedance of the antenna 215 in accordance with acontrol signal provided by the symbol generator 230. The modulator 220is shown in FIG. 2 implemented using a single field effect transistor.The gate of the field effect transistor may be coupled to the symbolgenerator 230 and receive a control signal from the symbol generator 230based on the data to be encoded into the backscatter signal. Otherdevices may be used to implement the modulator 220 in other examples.Such devices as a PIN diode, a varactor diode, a field effecttransistor, a bipolar transistor, or circuit combinations of theseelements may also be used to change the impedance state of the modulator220, and thus change the impedance of the load connected to antenna 215.

The symbol generator 230 may provide at least one subcarrier frequency.In some examples, only one subcarrier frequency may be provided by thesymbol generator 230. In some examples, multiple subcarrier frequenciesmay be provided. The symbol generator 230 may provide the subcarrierfrequency, for example, by having a frequency source that provides thesubcarrier frequency. For example, the symbol generator may have one ormore oscillators that may oscillate at the subcarrier frequency orsub-harmonics thereof. In some examples, the symbol generator may havemultiple frequency sources coupled to and/or included in the symbolgenerator and the symbol generator may select one of the multiplefrequency sources for use in providing the backscattered signal. Thesymbol generator may select one of the multiple frequency sources inaccordance with data provided to the symbol generator. For example, oneof the frequency sources may be used corresponding to a ‘0’ bit andanother of the frequency sources may be used corresponding to a ‘1’ bit.The phase and/or amplitude of the frequency sources may also be variedto produce a subcarrier frequency that is phase and/or amplitudemodulated.

The symbol generator 230 may control the modulator 220 to backscatter anincident signal having a carrier frequency (e.g. the signal 130 ofFIG. 1) using the subcarrier frequency to provide a backscattered signalat the antenna. By mixing the carrier frequency with the subcarrierfrequency or harmonics thereof, the backscattered signal may include abandpass signal in a predetermined frequency range. The predeterminedfrequency range may be specified by a combination of the carrier andsubcarrier frequencies.

In some examples, the backscatter device 200 may use sub-harmonic mixingto permit a carrier at a fraction of a desired band-pass signalfrequency to produce energy in the desired communication frequency band.In such embodiments, if the desired communication carrier frequency isat a frequency F_(carrier), the signal source (e.g. the signal source100 of FIG. 1) may be at a sub-harmonic frequency Fe r/n where n is aharmonic number. For example, an 800 MHz carrier may be used in asub-harmonic mode to generate backscatter energy in the 2.4 GHz band (inthis example, n=3) due to harmonic mixing in the backscatter device.

In some examples, the predetermined frequency range may be a rangespecified by a wireless communication protocol (e.g. a wirelesscommunication standard). For example, the wireless communicationprotocol may be Bluetooth Low Energy and the frequency range may be arange of an advertising channel specified by a Bluetooth Low Energyspecification.

Accordingly, the symbol generator 230 may control the modulator tomodulate the magnitude and/or phase of an incident signal to generate abackscattered signal. The backscattered signal may encode data, whichmay be provided to the symbol generator 230. The data may be, e.g. datacollected by a sensor or other device in communication with thebackscatter device 200. The data may be stored by the backscatter device200. Examples of the data include, but are not limited to, a temperatureof a portion of a building, an identity of an inventory item, atemperature of a food container, neural recording data, a biological orphysiological signal including measurement of a parameter relevant tohuman or animal health such as heart rate, blood pressure, bodychemistry such as oxygen level, glucose level, the level of anotheranalyte, or neural data such neural recording data or muscle activitysuch as electromyelogram or EMG data). For example, a neural recordingmay be relayed from a neural recording sensor (e.g. on an animalsubject, such as an insect, e.g. a dragonfly). The backscattered signalmay be formatted in accordance with a protocol expected by a wirelesscommunication device (e.g. a wireless communication standard).Accordingly, the backscattered signal may include a packet. The symbolgenerator 230 may control the modulator 220 to provide a packetformatted in accordance with a particular wireless communicationprotocol. The packet may include a preamble, an access address, apayload data unit, and a cyclic redundancy check.

The symbol generator may be implemented using hardware, software, orcombinations thereof. In some examples, the symbol generator 230 may beimplemented using a microprocessor.

In some embodiments the backscatter device 200 may include a processor(not shown in FIG. 2, but which processor may be in communication withthe symbol generator 230). The processor may be, for example,implemented using fixed-function digital logic (such as a finite statemachine or FSM) or a microprocessor or microcontroller which implementsoperations including memory and optional sensor inputs. In such examplesthe processor may encode a data stream including a unique identifier forthe backscatter device 200. Accordingly, the transmitted backscatteredsignal may include a unique identifier for the backscatter device 200.In some examples, the optional sensor inputs may influence one or morebits of the data stream in such a way as to encode the value of theoptional sensor inputs into the data stream by changing the uniqueidentifier that is sent. In such cases the aforementioned data streammay then be fed into the symbol generator as described herein and shownin FIG. 2.

In some examples, the processor formats the unique identifier, theoptional sensor input(s), and/or other data that is desired to be sentin the transmitted backscattered signal into a specified packet format,such as but not limited to a Bluetooth Low Energy advertising packet, anIEEE 802.11 beacon frame, an IEEE 802.15.4 beacon frame, or anotherspecified packet format. In such examples the packet format may thenform a data stream which may be provided to the symbol generator asdescribed herein. In some examples, such as in the case of the BluetoothLow Energy advertising packet, information about the channel on whichthe packet is being sent may be encoded in to the data stream itself. Insuch examples, the channel number may be derived from the parameters ofthe carrier frequency and the configuration of the symbol generator asdescribed herein.

In some examples, an incident signal from a signal source (e.g. thesignal source 100 of FIG. 1) may be bursty—e.g. the signal source mayprovide a packetized signal which is exploited by the backscatter device200 in generating a backscattered signal. Accordingly, in some examples,the signal source itself may be controlled to provide communicationsthat are as lengthy and/or continuous as possible. For example, when aBluetooth communication signal is provided by the signal source 100, thesignal source may provide packets sent with a maximum PDU length of 312bits. In some examples, the signal source may provide packets (e.g.Bluetooth packets) sequentially without gaps between the packets—forexample without gaps between the final CRC bit of one packet and thefirst preamble bit of a next packet. These techniques may increase anamount of time available during which a backscatter device may produce abackscatter signal in some examples. In some examples, the backscatterdevice (e.g. the backscatter device 110 of FIG. 1 and/or the backscatterdevice 200 of FIG. 2) may backscatter packets having a duration selectedto be transmitted within a time the signal source takes to provide apacket. So, for example, the signal source may provide a Bluetoothpacket having a PDU length of 312 and the backscatter device 200 mayhave a smaller PDU length such that the packet can be backscatteredduring the time the packet from the signal source is incident on thebackscatter device. The backscatter device may include a receiver thatmay receive the signal from the signal source and provide an indicationof when to start and/or stop backscattering based on presence, absence,or content of the signal 130 from the signal source 100.

In some examples, the signal source may provide packets which arespecifically chosen to yield advantageous properties which may beexploited by the backscatter device 200 in generating a backscatteredsignal. In some embodiments such advantageous properties may include thelength and/or duration of a packetized signal emitted by signal source100. In other embodiments, packets transmitted by signal source 100 maybe selected so as to increase and/or maximize energy in a preferredfrequency range. For example, in the case of signal source 100 using abinary frequency shift keying modulation, the packets transmitted by thesignal source may be constructed so as to favor the production of one ofthe two frequencies transmitted and reduce and/or minimize the number oftransitions between the two frequencies within a given packet. In thecase of a signal source 100 using a phase shift keying modulation, thepackets transmitted by the signal source may be constructed to reduceand/or minimize the number of phase transitions within a given packet.In the case of a signal source 100 using an orthogonal frequencydivision multiplexing (OFDM) signal, the packets transmitted by thesignal source may be constructed to improve and/or maximize energy in achosen OFDM subcarrier and minimize and/or reduce energy in other OFDMsubcarrier frequencies. Accordingly, the signal source 100 may provide asignal 130 whose features may be selected based on a number of frequencytransitions in the signal, a number of phase transitions in the signal,a number of amplitude transitions in the signal, and/or a number ofenergy-bearing subcarriers in the signal, or combinations thereof. Forexample, the signal may include a packet selected to minimize one ormore of (a) a number of frequency transitions in the incident signal,(b) a number of phase transitions in the incident signal (c) changes inamplitude of the incident signal, or (d) a number of energy bearingsubcarriers of the incident signal.

The backscatter device 200 may provide a transmitted backscatteredsignal compatible with an FSK or GFSK standard employed by a receivingwireless communication device. The symbol generator 230 may include orbe in communication with a frequency source that may be operated at oneof two frequency states, F_(mod1) or F_(mod2). The selection of thefrequency state may be made under the control of data that is input tothe symbol generator (e.g. from a sensor or microprocessor). Thefrequency source may include, for example, a resistance-capacitance (RC)oscillator, an inductance-capacitance (LC) oscillator, a quartz crystaloscillator, a frequency synthesizer, the output of a digital-to-analogconverter, the output of a direct digital synthesizer, the output of aclock generator, an arbitrary waveform generator, or any other analog ordigital frequency source or combinations thereof. In some examples, theaforementioned frequency source produces a square-wave output, and insome examples the aforementioned frequency source produces a sinusoidaloutput. In some examples the frequency source produces any waveformhaving energy at least including the frequency components F_(mod1) andF_(mod2). In some examples regulatory limits on occupied bandwidth orother properties of the signal may influence the choice of frequencysource waveforms.

In some examples, the sum of the frequency of an incident carrier F(e.g. the carrier received from the signal source 100 of FIG. 1), plusthe average of F_(mod1) and F_(mod2) is provided to be within anacceptable range (e.g. a range at which it may be correctly received bya receiving device communicating in accordance with that specification)of a channel center frequency signal specification. In such examples,x=F_(carrier)+mean(F_(mod1), F_(mod2)), where x is a frequency within anacceptable range of a channel center frequency signal specification. Insuch examples, the frequency source in the backscatter device may haveany waveform shape. In such examples, the difference between the twofrequencies, d1=abs(F_(mod1)−F_(mod2)), where abs( ) denotes theabsolute value operator, is provided to be within an acceptablefrequency deviation range of a frequency shift keying (FSK) or Gaussianfrequency shift keying signal specification.

In some examples, the difference y=F_(carrier)−mean(F_(mod1), F_(mod2))is provided to be within an acceptable range (e.g. a range at which itmay be correctly received by a receiving device communicating inaccordance with that specification) of a channel center frequency signalspecification. In such examples, the frequency source may have anywaveform shape. In such examples, the difference between the twofrequencies, d2=abs(F_(mod1)−F_(mod2)), where abs( ) denotes theabsolute value operator, is provided within an acceptable frequencydeviation range of a frequency shift keying (FSK) or Gaussian frequencyshift keying signal specification.

In some examples, harmonics of the backscatter signal may be used toform the transmitted backscattered signal. In such examples, theparameters F_(carrier), F_(mod1), and F_(mod2) are provided such that:z=F_(carrier)±(n×mean(F_(mod1), F_(mod2))), where n is a harmonic numberand z is within an acceptable range (e.g. a range at which it may becorrectly received by a receiving device communicating in accordancewith that specification) of a channel center frequency signalspecification. A further constraint on F_(mod1) and F_(mod2) may be thatthe frequency difference a=n×abs(F_(mod1)−F_(mod2)) is within anacceptable range (e.g. a range at which it may be correctly received bya receiving device communicating in accordance with that specification)of the frequency deviation specification. Thus the spacing betweenF_(mod1) and F_(mod2) may be reduced by a factor corresponding to theharmonic number employed, compared to the fundamental-mode where n=1.

In such examples, the frequency source may preferentially have a squarewave shape with n being an odd number, but any waveform shape havingenergy at the harmonics of F_(mod1) and F_(mod2) are possible. In theseembodiments the frequencies the difference between the two frequencies,d2=n×abs(F_(mod1)−F_(mod2)), where abs denotes the absolute valueoperator, is provided to be within an acceptable frequency deviationrange (e.g. a range at which it may be correctly received by a receivingdevice communicating in accordance with that specification) of afrequency shift keying (FSK) or Gaussian frequency shift keying signalspecification.

In some embodiments the frequency source switches nearly instantaneouslybetween F_(mod1) and F_(mod2) at a rate within an acceptable symbol raterange of a signal specification (e.g. a range at which it may becorrectly received by a receiving device communicating in accordancewith that specification). In other examples the frequency sourcetransitions smoothly between F_(mod1) and F_(mod2) over a period oftime, such that the transition is completed within an acceptable symbolrate range of a signal specification (e.g. a range at which it may becorrectly received by a receiving device communicating in accordancewith that specification). In some examples, the smooth transitionbetween F_(mod1) and F_(mod2) occurs according to a function of timesuch that the occupied bandwidth of the backscattered signal complieswith a regulatory or specification requirement. In some examples, thetransition is designed to produce a Gaussian frequency shift keyingspectrum. It should be understood that, while some examples herein referto a binary FSK or GFSK modulation scheme, including two modulatorfrequencies (F_(mod1) and F_(mod2)), other examples may include morethan two modulator frequencies, such as in m-ary FSK, where m refers toa number of frequency states. In such cases multiple modulatorfrequencies (F_(mod1) . . . F_(mod_m)) may be employed. The analogousconstraints on the choice of fundamental-mode and harmonic-modemodulator frequencies would be applied as described herein.

In some examples, an incident signal provided by a source device may bea signal in one channel in accordance of a wireless communicationstandard (e.g. a Bluetooth signal) and may be backscattered by thebackscatter device 200 into another channel of the wirelesscommunication device. For example, a source device may provide a signalon Bluetooth channel 38 and the backscatter device 200 may backscatterthe signal on Bluetooth channel 38 into a backscatter signal onBluetooth channel 37 and/or 39. Other channels may be used in otherexamples. Furthermore a source device may produce a signal using a firstwireless communication standard which the backscatter device retransmitsin a manner compatible with a second wireless communication standard.

The backscatter device 200 may provide a transmitted backscatteredsignal compatible with phase shift keying (PSK) standard employed by areceiving wireless communication device. The PSK standard may have atleast one or more specified phase differences, one or more specifiedchannel center frequencies and one or more specified symbol rates.

In examples utilizing PSK, the symbol generator 230 may include afrequency source that may be operated at a frequency F_(mod) with one ofat least two phase states, P_(mod1), P_(mod2) through P_(mod_n). Theselection of the phase state is made under the control of a data streaminput to the symbol generator. For example, one phase state may beselected corresponding to a ‘0’ bit and another phase state selectedcorresponding to a ‘1’ bit. In some examples, such as in the binaryphase shift keying (BPSK) case, P_(mod1) and P_(mod2) differ by 180degrees (pi radians). In other embodiments, such as n-PSK where n is thenumber of different phase states, multiple different phases may beemployed.

The frequency source may include a resistance-capacitance (RC)oscillator,

an inductance-capacitance (LC) quartz crystal oscillator, a frequencysynthesizer, the output of a digital-to-analog converter, the output ofa direct digital synthesizer, the output of a clock generator, anarbitrary waveform generator, or any other analog or digital frequencysource. In some examples, the aforementioned frequency source produces asquare-wave output, while in other embodiments the aforementionedfrequency source produces a sinusoidal output.

In some examples the frequency source produces any waveform havingenergy at least including the frequency component F_(mod) with a phasethat can be varied from P_(mod1) to P_(mod2) through P_(modn).

In some examples, the sum of the frequency of an incident carrierF_(carrier), plus the F_(mod), is provided within an acceptable range(e.g. a range at which it may be correctly received by a receivingdevice communicating in accordance with that specification) of a channelcenter frequency signal specification, such that x=F_(carrier)+F_(mod).In such embodiments, the frequency source may have any waveform shape.In such examples, the difference between the phase states P_(mod1),P_(mod2) through P_(modn), is selected to be within an acceptable phaseshift range (e.g. a range at which it may be correctly received by areceiving device communicating in accordance with that specification) ofa phase shift keying (PSK) signal specification.

In some embodiments, the difference y=F_(carrier)−F_(mod) is provided tobe within an acceptable range (e.g. a range at which it may be correctlyreceived by a receiving device communicating in accordance with thatspecification) of a channel center frequency signal specification. Insuch examples, the frequency source may have any waveform shape. In suchexamples, the difference between the phase states P_(mod1), P_(mod2)through P_(modn), is selected to be within an acceptable phase shiftrange (e.g. a range at which it may be correctly received by a receivingdevice communicating in accordance with that specification) of a phaseshift keying (PSK) signal specification.

In some examples, the parameters F_(carrier) and F_(mod) are providedsuch that: z=F_(carrier)±(n×F_(mod)), where n is a harmonic number, andz is within an acceptable range (e.g. a range at which it may becorrectly received by a receiving device communicating in accordancewith that specification) of a channel center frequency signalspecification. In such embodiments, the frequency source maypreferentially have a square wave shape with n being an odd number, butany waveform shape having energy at the harmonics of F_(mod) arepossible.

In some embodiments the frequency source switches nearly instantaneouslybetween phase states at a rate within an acceptable symbol rate range(e.g. a range at which it may be correctly received by a receivingdevice communicating in accordance with that specification) of a signalspecification. In some examples, the frequency source transitionssmoothly between phase states over a period of time, such that thetransition is completed within an acceptable symbol rate range (e.g. arange at which it may be correctly received by a receiving devicecommunicating in accordance with that specification) of a signalspecification. In some embodiments, the smooth transition between phasestates occurs according to a function of time such that the occupiedbandwidth of the backscattered signal complies with a regulatory orspecification requirement.

The backscatter device 200 may provide a transmitted backscatteredsignal compatible with amplitude shift keying (ASK) standard employed bya receiving wireless communication device. The ASK standard may have aspecified modulation depth, one or more specified channel centerfrequencies, and one or more specified symbol rates.

To implement amplitude shift keying of the reflected or scatteredsignal, two example implementations are described. In examples using100% modulation depth amplitude shift keying (e.g. sometimes calledon-off keying or OOK), one input to the modulator (e.g. modulator 220 ofFIG. 2) may be provided. This input may be toggled (e.g. by the symbolgenerator 230) at a first frequency F_(modsc) for one symbol period whenthe symbol to be sent is e.g. a “1”. The input is held off (e.g. nottoggled) for one symbol period when the symbol to be sent is e.g. a “0”.

In some examples, at least two inputs to the backscatter modulator areprovided to permit amplitude shift keying with other than 100%modulation depth. A first input is used to generate a subcarrierfrequency by switching the modulator 220 at a first frequency F_(modsc).A second input is used to vary the modulation depth of the subcarrierfrequency at the symbol rate. One method for varying the modulationdepth is to add a resistor in parallel with the aforementioned modulator220, such as a resistor with a resistance R in parallel with theswitching FET. In such an embodiment, the real part of the impedancepresented to the antenna then varies between the real part of theparallel circuit with the transistor off (R//transistor impedance_off)and the real part of the parallel circuit with the transistor on(R//transistor impedance_on). The backscattered subcarrier will thenhave two different modulation depths when the second input is switchedbetween a “1” and a “0” in accordance with the data to be sent.

In examples using ASK, the symbol generator 230 may include a subcarrierfrequency source that may be operated at a frequency F_(modsc). Thefrequency source may include a resistance-capacitance (RC) oscillator,an inductance-capacitance (LC) oscillator, a quartz crystal oscillator,a frequency synthesizer, the output of a digital-to-analog converter,the output of a direct digital synthesizer, the output of a clockgenerator, an arbitrary waveform generator, or any other analog ordigital frequency source. In some examples, the aforementioned frequencysource produces a square-wave output, while in other embodiments theaforementioned frequency source produces a sinusoidal output. In someexamples the frequency source produces any waveform having energy atleast including the frequency component F_(modsc).

In some examples, the sum of the frequency of an incident carrierF_(carrier), plus the F_(modsc) is provided to be within an acceptablerange (e.g. a range at which it may be correctly received by a receivingdevice communicating in accordance with that specification) of a channelcenter frequency signal specification, such thatx1=F_(carrier)+F_(modsc). In such examples, the frequency source mayhave any waveform shape.

In some examples, the difference y1=F_(carrier)−F_(modsc) is selected tobe within an acceptable range (e.g. a range at which it may be correctlyreceived by a receiving device communicating in accordance with thatspecification) of a channel center frequency signal specification. Insuch examples, the frequency source may have any waveform shape.

In some examples, the parameters F_(carrier) and F_(modsc) are chosensuch that: z=F_(carrier)±(n×F_(modsc)), where n is a harmonic number,and z is within an acceptable range (e.g. a range at which it may becorrectly received by a receiving device communicating in accordancewith that specification) of a channel center frequency signalspecification. In such embodiments, the frequency source maypreferentially have a square wave shape with n being an odd number, butany waveform shape having energy at the harmonics of F_(modsc) arepossible.

In some examples the modulator switches nearly instantaneously betweentwo modulation depth states at a rate within an acceptable symbol raterange (e.g. a range at which it may be correctly received by a receivingdevice communicating in accordance with that specification) of a signalspecification. In other embodiments the frequency source transitionssmoothly between modulation depth states over a period of time, suchthat the transition is completed within an acceptable symbol rate range(e.g. a range at which it may be correctly received by a receivingdevice communicating in accordance with that specification) of a signalspecification. In some embodiments, the smooth transition betweenmodulation depth states occurs according to a function of time such thatthe occupied bandwidth of the backscattered signal complies with aregulatory or specification requirement.

The backscatter device 200 may provide a transmitted backscatteredsignal compatible with orthogonal frequency division multiplexing (OFDM)standards employed by a receiving wireless communication device.Generally, techniques described herein for providing backscattered FSK,PSK, QAM, and ASK signals may be extended to produce OFDM signals.

In examples using OFDM, multiple band-pass signals may be generated bythe modulator 220, one such bandpass signal per OFDM subcarrier. Thismay be implemented by providing multiple modulator frequencies such thattheir fundamental mode and/or harmonic mode frequency components alignwith the subcarrier spacing specified for the OFDM standard and/ortechnique. Each of the OFDM subcarriers may be modulated with e.g. a PSKsignal per the description herein for PSK modulation examples. Themultiple modulator frequencies may be applied to the same modulator(e.g. transistor). In some examples, a non-linear mixing operation maybe implemented using a logic combination of the multiple modulatorfrequencies such as an exclusive-or (XOR) gate or an OR gate.

In some examples, a linear operation may be employed via an analog powercombination of the multiple modulator frequencies provided to themodulator 220.

In some examples, the backscatter device 200 may harvest at least partof its operating power from the environment, for example using anoptional RF energy harvesting circuit that may be included in and/orco-located with the backscatter device 200. In some embodiments thisenergy may be used directly by the backscatter device 200, while inother embodiments this energy may be stored in a reservoir such as butnot limited to a capacitor, a supercapacitor, or a battery that isincluded in and/or co-located with the backscatter device 200. In suchexamples harvested energy may be accumulated in the reservoir for aperiod of time and then released to the operating circuitry of thebackscatter device 200. This may be either a predetermined period oftime, or a period of time corresponding to a time at which the reservoirreaches a particular amount of stored energy.

FIG. 3 is a flowchart illustrating a method in accordance with examplesdescribed herein. The method includes receiving a signal having acarrier frequency in block 305, backscattering the signal to provide abackscattered signal in block 310. Block 310 may include modulatingimpedance of at least one antenna in accordance with data (block 315)and mixing the carrier frequency with a subcarrier (block 320). Themethod 300 also including transmitting the backscattered signal in block325.

Block 305 may be implemented, for example, by the backscatter device 200of FIG. 2 or the backscatter device 110 of FIG. 1 receiving a signalhaving a carrier frequency (e.g. the signal 130 of FIG. 1) at theirantenna. Block 310 may be implemented, for example, by the backscatterdevice 200 of FIG. 2 or the backscatter device 110 of FIG. 1. Forexample, the symbol generator may receive data and control the modulatorof FIG. 2 to modulate impedance of the antenna 215 of FIG. 2 in block315. This process may involve mixing the subcarrier frequency with thecarrier frequency.

The backscattered signal may then be transmitted in block 325 toproduce, for example, the transmitted backscatter signal 135 of FIG. 1.

The signal having a carrier frequency may be backscattered in block 310to provide a backscattered signal. This may include mixing the carrierfrequency with a subcarrier in block 320. The mixing in block 320 mayresult in a bandpass signal having a predetermined frequency range. Thepredetermined frequency range may be a range of an advertising channelin accordance with a wireless communication standard, such as theBluetooth Low Energy standard.

Backscattering the signal in block 310 may include modulating impedanceof at least one antenna in accordance with data to be provided in thebackscattered signal. Modulating may include reflecting the signal in apattern indicative of the data to be provided in the backscatteredsignal. The data to be provided in the backscattered signal may includea packet having a preamble, an access address, a payload data unit, anda cyclic redundancy check. The data may include, for example, anindication of a temperature associated with a device providing thebackscattered signal, or an identification of an asset associated with adevice providing the backscattered signal.

Packets that may be provided in accordance with examples describedherein include, but are not limited to, Bluetooth Low Energy advertisingpackets, IEEE 802.11 beacon frames, and IEEE 802.15.4 beacon frames.

Furthermore, it should be appreciated that the backscatter device 200may employ one or more of the methods disclosed herein to generate acoded modulation such as a polyphase coded sequence of chips or symbolsformed from one or more of the ASK, PSK, or QAM signals as describedherein. One example of the polyphase coded sequences that may begenerated by the backscatter device 200 may be a complementary codekeying (CCK) sequence.

FIG. 5 is a schematic illustration of a backscatter device arranged inaccordance with examples described herein. The backscatter device 502may be used to implement and/or be implemented by the backscatter device110 of FIG. 1 and/or the backscatter device 200 of FIG. 2 in someexamples. The backscatter device 502 includes a controller 504,frequency synthesizer 506, analog switch 508. RF switch 510, and antenna512.

The RF switch 510 may be coupled to antenna 512 and may modulate animpedance of the antenna in order to backscatter an incident signal, thesignal 130 provided by a signal source in FIG. 1. The RF switch 510 mayprovide subcarrier modulation using a subcarrier frequency. In someexamples, one of multiple frequencies may be selected—e.g. onecorresponding to a ‘0’ bit and another corresponding to a ‘1’ bit.Accordingly, in some examples an analog switch 508 may be provided toselect between multiple frequencies provided by one or more frequencysynthesizers, such as frequency synthesizer 506. While a singlefrequency synthesizer 506 is shown in FIG. 5, multiple frequencysynthesizers may be provided in other examples. A controller 504 may becoupled to the analog switch 508 and, in some examples, the frequencysynthesizer 506. The controller 504 may provide data (e.g. data fortransmission in a backscattered signal) to control the analog switch508. The controller 504 may provide control signals (e.g. an indicationof one or more frequencies for use in backscatter), to the frequencysynthesizer 506. For example, the controller 504 may be used to programthe frequency synthesizer 506 to produce one or more particularfrequencies for sub-carrier modulation.

The antenna 512 may be implemented using any of a variety of antennasincluding, but not limited to, a chip antenna, a trace antenna (e.g.antennas integrated into a chip including the RF switch 510 and/oranalog switch 508), a dipole antenna, or combinations thereof. Otherantennas may also be used.

It may be desirable to have a phase continuous transition betweenmultiple subcarrier modulation frequencies (e.g. the two frequenciesprovided by the frequency synthesizer 506). For example, at thetransition between a “0” and a “1” data bit the modulation signal shouldmaintain its phase. In some examples, the modulation signal may bedirectly digitally synthesized using an arbitrary waveform generator. Insome examples, however, coordination between subcarrier signals may beused. For example, the frequency synthesizer 506 may provide a number(e.g. 2, 3, 4, 5, 6, 7, 8, 9, 10) of output frequencies from a commonfrequency reference. By using harmonics of the data rate at which thebackscatter signal will be provided (e.g. the BLE data), a constantphase may be maintained between the multiple (e.g. two) subcarrierfrequencies at the time of bit transitions. For example, by choosingsubcarrier frequencies spaced apart by a BLE data rate a continuousphase at the transition times may be provided.

EXAMPLE

These detailed examples of systems operating in accordance with aBluetooth Low Energy specification are provided to facilitateunderstanding, although it is not intended to be limiting, nor toindicate that these were the only detailed example investigated,contemplated, or implemented.

In one example a system provides interoperability between a backscatterdevice and a wireless communication device having a Bluetooth Low Energychipset as is commonly found in mobile devices such as tablet computers,such as the APPLE iPAD or SAMSUNG GALAXY tablets or smart phones such asthe APPLE IPHONE or SAMSUNG GALAXY series.

The Bluetooth Low Energy (BTLE) specification details a wirelesscommunication scheme using Gaussian frequency shift keying with achannel specification of 40 channels with center frequencies rangingfrom 2402 MHz to 2480 MHz. The data rate is 1.0 Mbps while the channelspacing is 2.0 MHz. The minimum frequency deviation is 185 kHz.

Three of the 40 channels, channels 37, 38, 39, with center frequenciesof 2402 MHz, 2426 MHz, and 2480 MHz respectively, are referred to asadvertising channels. A conventional Bluetooth Low Energy device listenson each of the advertising channels in turn to identify nearby BTLEdevices.

In one example, the modulator (e.g. modulator 220 of FIG. 2) isimplemented using a type BF1108R field effect transistor manufactured byNXP, Inc. The symbol generator (e.g. symbol generator 230 of FIG. 2) isimplemented using an Agilent 335008 arbitrary waveform generator. Thesource and drain terminals of the field effect transistor are connectedto a first dipole antenna resonant near 2450 MHz. The signal source(e.g. signal source 100 of FIG. 1) is implemented using an AgilentN5181A signal generator set to a desired carrier frequency F_(carrier),with an output power of +15 dBm, connected to a second dipole antennaresonant near 2450 MHz. The wireless communication device receiving thecommunication (e.g. the wireless communication device 120 of FIG. 1) isimplemented using an APPLE IPAD running APPLE IOS with the Pally BLEscanner application.

In one example, the signal source (e.g. the signal source 100 of FIG. 1)is implemented as supplying a dedicated carrier source with a frequencyof F_(carrier)=2453 MHz. To produce a bandpass signal in the Channel 38and 39 passbands, modulating frequencies F_(mod1)=26.7 MHz andF_(mod2)=27.3 MHz. The sum of the carrier frequency plus the mean of thetwo modulation frequencies is 2453 MHz+mean(26.7 MHz, 27.3 MHz)=2480 MHz(Channel 39), leveraging the upper sideband modulation, while thedifference between the carrier frequency and the mean of the twomodulation frequencies is 2453 MHz−mean(26.7 MHz, 27.3 MHz)=2426 MHz(Channel 38), leveraging the lower sideband modulation. Thus a singlecarrier frequency may serve multiple channels using the previouslymentioned fundamental mode approach. Note that in this implementationthe frequency deviation between the two modulation frequencies is 600kHz which complies with the required minimum frequency deviationspecification of 185 kHz.

To produce a bandpass signal in the Channel 37 passband of 2402 MHz, asecond harmonic (n=2) approach may be used. In this case, modulatingfrequencies F_(mod1)=25.35 MHz and F_(mod2)=25.65 MHz. Thus the secondharmonic band-pass signal falls at 2453 MHz-2*mean(25.35 MHz, 25.65MHz)=2402 MHz. Note that in this implementation the difference betweenthe two modulation frequencies is only 300 kHz to yield asecond-harmonic difference of 600 kHz which complies with the minimumfrequency deviation specification of 185 kHz.

In this manner, all three advertising channels may be addressed usingonly a single carrier frequency of 2453 MHz. The modulating frequenciesare in the range of 25.35 MHz to 27.3 MHz which is far lower than thecarrier frequency. Thus the backscatter device may consume far lesspower than would be required to generate the carrier frequency. In thisexample the symbol generator comprises an Agilent 33500B arbitrarywaveform generator using a waveform synthesized as a sampled vectorusing the MATLAB signal processing toolbox. The symbol rate is 1.0 Msps.

In this example, the symbol generator (e.g. symbol generator 230 of FIG.2) is produces a packet format compatible with the BTLE specification.FIG. 4 is a schematic illustration of an example packet compatible withthe BTLE specification. From the least significant bit to the mostsignificant bit, an example packet includes the preamble OxAA, theaccess address Ox8E89BED6, the payload data unitOx43C80844210712140201050909456E73776F727468, and the cyclic redundancycheck Ox9BC70A. The MATLAB signal processing toolbox encodes the packetusing the data whitening function specified in the BTLE specification. Adifferent data whitening function input is used for communication overeach channel. For each bit in the encoded packet, the modulatingfrequency is set to F_(mod1) if the corresponding bit is “0” andF_(mod2) if the corresponding bit is “1”. The packet shown in FIG. 4 mayaccordingly be transmitted as, for example, the transmitted backscattersignal 135 of FIG. 1.

In another example, a backscatter tag was developed that producesthree-channel band-pass frequency shift keying (FSK) packets at 1 Mbpsthat are indistinguishable from conventional BLE advertising packets.Communication in all three of the BLE advertising channels was performedusing a single incident continuous wave (CW) carrier and a combinationof fundamental-mode and harmonic-mode backscatter subcarrier modulation.Further, a microcontroller-based backscatter tag capable of producingBLE advertising packets was demonstrated. Ranges of up to 18 meters weredemonstrated between the CW carrier source and BLE receiver. In theseexamples, the BLE receiver was implemented using an unmodified AppleiPad mini using its existing iOS Bluetooth stack with no modificationswhatsoever to hardware, firmware, or software.

Additionally, BLE advertising packets were generated with a non-CWcarrier. Backscattered BLE packets were demonstrated using a BLE signalas a carrier source. Reception of the backscattered BLE messages wasdemonstrated with two unmodified BLE devices, an Apple iPad mini and ageneric PC equipped with a Nordic Semiconductor nRF51882 BLE chipset.Each device successfully demodulated and accepted the advertisingmessages and passed them up their Bluetooth stacks without anymodifications. Successful reception was shown at a range of 18 metersusing a +23 dBm EIRP fixed frequency source and an unmodified Apple iPadmini.

A single FET was used as an RF switch (e.g. the RF switch 510 of FIG. 5)and an Agilent 33500B arbitrary waveform generator as the basebandmodulation source. The backscatter device operated by modulating theload impedance presented to the antenna based on the baseband modulationsignal. The test board itself included a single NXP BF1108R FET with thegate voltage controlled by an arbitrary waveform generator, the drainconnected to ground, and the source connected to an SMA port with a 2.4GHz dipole antenna. Messages were sent sequentially to channel 37,channel 38, and channel 39 with payload data units (PDUs) containingdevice names Alice, Bob, and Charlie. The control signal from thewaveform generator was created in Matlab with a 220 MSa/s sampling rateto ensure that the 27 MHz subcarrier frequencies were faithfullyreproduced. The sample vector was transferred to the arbitrary waveformgenerator via a USB stick. Three individual control vectors wereconcatenated, one for each of the three advertising channels. A sequencecontrolling the time varying subcarrier selection for a single channelwas devised in accordance with the data, e.g. for a message to be senton channel 39 with data “Charlie”. A “1” bit was transferred with apositive subcarrier deviation and a “0” bit was transferred with anegative subcarrier deviation.

In an over the air (OTA) test with the BF1108R test board an unmodifiedApple iPad running the BLE Scanner app was used to successfully receivethe three messages, Alice, Bob, and Charlie from a single CW carrier.The separation distance between the CW carrier and Apple iPad was 9.4meters, and tests were performed with the BF1108R test board at severallocations between the carrier and iPad.

In an example of a backscatter device that does not rely on testequipment for a modulation source, an implementation that produces twosubcarrier frequencies derived from a common frequency reference sourcewas demonstrated. Selecting which of the two frequencies are used todrive a modulating FET (e.g. the RF switch 510 of FIG. 5) controlledwhether a BLE “0” or “1” was being backscattered. An Atmel ATmega328microcontroller on board an Arduino Nano was used to control which ofthe two subcarrier frequencies are driving the modulating FET (e.g. usedto implement the controller 504 of FIG. 5). The use of an Arduino Nanofacilitated programming the Atmel ATmega328 microcontroller. Thesubcarrier frequencies used in the BLE backscatter tag were produced bya Texas Instruments CDCE913 frequency generator chip used to implementthe frequency synthesizer 506 of FIG. 5. The CDCE913 is a programmablecomponent capable of producing output frequencies up to 230 MHz.

The Arduino Nano's I2C bus was used to program the frequency synthesizerto produce the desired subcarrier frequencies. Programming the frequencysynthesizer required a one time write to the synthesize's memoryregisters. The Arduino Nano was also used to drive a Texas InstrumentsSN74LVC1G3157 analog switch (e.g. used to implement the analog switch508 of FIG. 5) controlling which of the sub-carrier frequencies drivethe modulating FET (e.g. the RF switch 510 of FIG. 5). An N channel dualgate MOSFET, NXP BF1212, connects the tag antenna to either an open orshort circuit depending on the modulation control signal. In otherexamples a different type of RF switching device may be connected to thetag antenna, such as a CMOS RF switch, for example the ADG918 CMOS RFswitch manufactured by Analog Devices Inc.

The backscatter device had an SMA connector so a variety ofconnectorized antennas could be tested. Generally, if a smaller formfactor is desired the antenna could be integrated into the PCB with achip antenna, a patch antenna, or a trace antenna. If an applicationscenario calls for placement on or near a high dielectric surface theantenna could be specifically designed for that case as well. For thetesting described in this specific example a dipole antenna was used.

To construct a BLE compatible signal it may be desirable to have a phasecontinuous transition between the two subcarrier modulation frequencies.At the transition between a “0” and a “1” data bit the modulation signalshould maintain its phase. This could be accomplished with directdigital synthesis of the modulation signal as was done with the testequipment implementation using an arbitrary waveform generator. Tocreate phase continuous transitions in the stand alone implementation,coordination between the subcarrier signals was used. Two independentfree running oscillators were not used because they have no common phaserelationship. The CDCE913 chip is capable of producing three separateoutput frequencies all derived from a common frequency reference input.By using harmonics of the BLE data rate we maintain a constant phasebetween the two subcarrier frequencies at the time of bit transitions.In an example bit transition, the BLE data rate is 1 Mbps meaning thatever 1 uSec there is a possibility for a bit transition and a switchingfrom one subcarrier frequency driving the modulating FET to the othersubcarrier. By choosing subcarrier frequencies spaced apart by the BLEdata rate we can achieve a continuous phase at the transition times.Only the fundamental mode was used with messages being sent to BLEadvertising channels 37 and 38. The message sent on channel 37 containeddevice name Alice and the message sent on channel 38 contained devicename Bob. A single continuous wave frequency of 2414.5 MHz was used asthe incident signal from a signal source. The lower side band (LSB)subcarrier frequencies were f_(sc,0)=12.5+0.5=13 MHz andf_(sc,1)=12.5−5=12 MHz. The upper side band (USB) frequencies weref_(sc,0)=11.5−0.5=11 MHz and f_(sc,1)=11.5+0.5=12 MHz. This resulted inbackscattered signal present in Channel 37 (e.g. centered at 2402 MHz,with one sideband at 2401.5 and another at 2402.5 MHz) and Channel 38(e.g. centered at 2426 MHz with one sideband at 2426.5 MHz and anotherat 2425.5 MHz). For testing two separate tags were used, onebackscattering Alice messages and another backscattering Bob messages.Both tags used a packet length of 232 bits meaning their backscatteringduration was 232 μS. The tags were constantly backscattering duringtesting with a 500 mS delay between packets.

In some examples, a data-carrying signal itself (e.g. a Bluetoothsignal) may be used as the incident signal from a signal source to bebackscattered by backscatter devices described herein. The Bluetooth 4.0Low Energy spec has channels with a bandwidth of 2 MHz and a requirementfor the subcarrier spacing of only ±185 kHz, which leaves a significantrange of frequencies that are assigned to either a “1” or “0” data bit.This opens the possibility for using a conventional BLE signal as thecarrier while still having the backscattered signal (e.g. message) fallwithin the BLE spec. Unlike typical backscatter systems that use asingle frequency containing no data, this is an example of modifying acommunication signal containing data and inserting a new message with abackscatter device.

With this scheme a smart phone or pair of smart phones may act as boththe carrier source and the receiver. For a single device to operate asboth the carrier and receiver the BLE chipset may operate in full duplexmode, simultaneously transmitting in one channel and receiving inanother. In some examples, a single device may operate as the carrierfor a backscatter device and any other BLE enabled devices in the areato receive both the BLE carrier message and the BLE backscatteredsignal.

The backscattered packet may be designed in the same way as describedherein. Note that when a data-carrying (e.g. a communication signal) isused as the carrier instead of a continuous wave signal, the brieftransmission time of the incident signal may be accounted for—e.g. thebackscatter device should backscatter during a time that the signalsource is providing an incident signal. When the incident signal is apacketized signal, it may be bursty. To create a carrier BLE messagethat is longer than the backscatter window we can take advantage of theBLE advertising packet's variable length PDU. A single conventionalpacket could be sent with the maximum PDU length, 312 bits, and thebackscatter message may have a reduced PDU length so the backscatterwindow falls inside the time to transmit a conventional packet.Alternatively, and as was done in this example, multiple conventionalBLE packets can be broadcast sequentially without gaps between the finalCRC bit of one message and the first preamble bit of the next message.

In one example multiple conventional BLE packets were transmitted withno gaps between the end of one transmission and the beginning of thenext. The BLE packets were transmitted each containing the message Bob.At some time while the sequence of Bob messages was being broadcast anAlice packet was backscattered using the Bob transmissions as a carrier.There was no data coordination between the conventional source packetand the backscattered packet. The conventional transmission could havecontained any message, the use of Bob messages was only to show thatboth the conventional message and backscattered message can be receivedby an unmodified receiver in an Apple iPad mini.

Another factor is how an FSK receiver interprets the backscatteredsignal. If a conventional BLE message is modulated such that thesubcarrier frequency deviations are kept close to the minimum requiredin the BLE spec, 185 kHz, a message from one channel can bebackscattered to a new channel containing entirely new data. Theoriginal frequency deviation is still present in the backscatteredsignal but it may become a deviation contained entirely in the “0” or“1” frequency range as determined by the backscatter modulation. In someembodiments, the original frequency deviation may be minimized bycausing the signal source 100 to transmit specially chosen packets whichminimize the number of frequency transitions in the packet and thusminimize the effective original frequency deviation.

For example, a conventional BLE message was created for channel 38 andthat message was then backscattered to channel 37. This method maygenerally work with any channel combination. The message on channel 38may contain device name “Bob and the backscattered message on channel 37may contain device name “Alice.” (e.g. the data on the source andbackscattered signals may be unrelated). The conventional source signalwas constantly broadcasting in channel 38 and it was switching between 2FSK frequencies, 2426.75 MHz and 2426.25 MHz at a 1 Mbps data rate. Thisis similar in concept to using a continuous wave signal that driftsquickly in time or has high phase noise. The exact carrier frequency maynot be known but the range may be specified and that range may be narrowenough to still be useful.

FIGS. 6A-6C are schematic illustrations of spectra of the source signalin Channel 38 (FIG. 6A) and the backscattered ‘0’ signal in Channel 37(FIG. 6B) and the backscattered ‘1’ signal in Channel 37 (FIG. 6C). TheBLE transmission designed for channel 38 was backscatter-modulated withsubcarrier frequencies of 24 MHz±500 kHz. In FIG. 6A, the channel 38source signal is shown centered at 2426 MHz with sidebands at + and −250kHz. The 500 kHz frequency deviation is able to shift either of thechannel 38 frequencies to the “0” or “1” frequency range for channel 37.Since the carrier and backscattered message are both BLE packets thedata rate, 1 Mbps, is the same for both. That means there will be atmost one carrier frequency deviation per bit in the backscatteredsignal. For example, over 1 uSec, the duration of 1 bit of thebackscatter message, if the carrier message transitions from a “0” to a“1” and the backscatter data bit is a “1” the receiver will see first2402.25 MHz and then 2402.75 MHz. Both 2402.25 MHz and 2402.75 MHz areinterpreted by the receiver as a “1” data bit. The backscattered ‘O’signal in Channel 37 is shown in FIG. 6B. Channel 37 has a centerfrequency of 2402 MHz. The backscattered Channel 38 signal from FIG. 6Abackscattered as a ‘0’ may provide signal (shown in FIG. 6B) at 2402MHz−250 kHz=2401.75 MHz and 2402 MHz−750 kHz=2401.25 MHz, both of whichmay be interpreted as a ‘0’. The backscattered Channel 38 signal fromFIG. 6A backscattered as a ‘1’ may provide signal (shown in FIG. 6C) at2402 MHz+250 kHz=2402.25 MHz and 2402 MHz+750 kHz=2402.75 MHz, both ofwhich may be interpreted as a ‘1’.

From the foregoing it will be appreciated that although specificembodiments of the technology have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the technology. Also, in some embodiments themicrocontroller can be omitted, or the battery can be larger. Further,certain aspects of the new technology described in the context ofparticular embodiments may be combined or eliminated in otherembodiments. Moreover, while advantages associated with certainembodiments of the technology have been described in the context ofthose embodiments, other embodiments may also exhibit such advantages,and not all embodiments need necessarily exhibit such advantages to fallwithin the scope of the technology. Accordingly, the disclosure andassociated technology can encompass other embodiments not expresslyshown or described herein.

What is claimed is:
 1. A backscatter device comprising: an antennaincluding a load impedance; and an energy harvesting circuit configuredto harvest energy from radio frequency (RF) signals incident on theantenna; a waveform generator configured to generate two or moresubcarrier frequencies, wherein the waveform generator comprises afrequency synthesizer configured to provide a number of outputfrequencies from a frequency reference to generate the two or moresubcarrier frequencies; a modulator configured to switch the loadimpedance of the antenna to provide a backscattered signal using atleast a portion of the RF signals incident on the antenna; and an analogswitch coupled between the waveform generator and the modulator andconfigured to switch among the two or more subcarrier frequencies. 2.The backscatter device of claim 1, wherein the modulator is furtherconfigured to mix the portion of the RF signals incident on the antennawith a subcarrier frequency to provide the backscattered signal.
 3. Thebackscatter device of claim 2, wherein the backscattered signalcomprises a packet formatted in accordance with a Bluetooth standard. 4.The backscatter device of claim 1, further comprising: an energyreservoir configured to accumulate harvested energy for a period oftime, wherein the energy harvesting circuit is configured to storeharvested energy in the energy reservoir.
 5. The backscatter device ofclaim 4, wherein the energy reservoir comprises at least one of acapacitor, supercapacitor, or a battery.
 6. The backscatter device ofclaim 4, wherein the period of time corresponds to a predeterminedperiod of time or a time at which the energy reservoir reaches aparticular amount of stored energy.
 7. The backscatter device of claim1, wherein the RF signals incident on the antenna comprise an orthogonalfrequency division multiplexing (OFDM) signal including packetsconstructed to maximize energy in a particular OFDM subcarrier andminimize a number of energy-bearing OFDM subcarriers.
 8. The backscatterdevice of claim 1, wherein the energy harvesting circuit is furtherconfigured to extract at least some portion of the operating power forthe backscatter device from the RF signals incident on the antenna,including the operating power for the modulator.
 9. The backscatterdevice of claim 1, wherein the energy harvesting circuit is furtherconfigured to harvest energy from at least one environmental signal ofan environment of the backscatter device.
 10. The backscatter device ofclaim 9, wherein the at least one environmental signal comprises acontinuous wave signal.
 11. A method comprising: harvesting energy froman radio frequency (RF) signal, the RF signal comprising at least one ofa television transmission signal, a cellular communication signal, aWiFi signal, a Bluetooth signal, or a Zigbee signal; generating one ormore subcarrier frequencies, wherein generating the one or moresubcarrier frequencies comprises providing a number of outputfrequencies from a frequency reference, wherein the number of outputfrequencies from the frequency reference are representative of frequencyharmonics for a data rate of a Bluetooth Low Energy (BLE) signal; andmodulating, by a switch, a load impedance of an antenna with the RFsignal incident upon the antenna to provide a backscattered signal. 12.The method of claim 11, wherein the backscattered signal comprises apacket formatted in accordance with a Bluetooth standard.
 13. The methodof claim 11, further comprising: mixing a carrier frequency of the RFsignal with at least one of the one or more subcarrier frequencies. 14.The method of claim 11, wherein harvesting energy from the RF signalcomprises: extracting at least some portion of an operating power forthe switch from the RF signal.
 15. The method of claim 11, furthercomprising: accumulating, in a capacitor of a backscatter device, theenergy from the RF signal for a period of time at which the capacitorreaches a particular amount of stored energy.
 16. An apparatuscomprising: an antenna; a waveform generator configured to generate oneor more subcarrier frequencies, wherein the waveform generator comprisesa frequency synthesizer configured to provide a number of outputfrequencies from a frequency reference to generate the one or moresubcarrier frequencies, and wherein the number of output frequenciesfrom the frequency reference are representative of frequency harmonicsfor a data rate of a Bluetooth Low Energy (BLE) signal; and a modulatorconfigured to adjust a frequency of an incident signal at the antenna,the incident signal having a carrier frequency, the modulator configuredto adjust the frequency of the incident signal using at least one of theone or more subcarrier frequencies to provide a backscattered signal.17. The apparatus of claim 16, wherein the incident signal comprises aBluetooth signal.
 18. The apparatus of claim 16, wherein thebackscattered signal comprises a Bluetooth advertising packet comprisinga sensor ID and associated sensor data.
 19. An apparatus comprising: anantenna; a waveform generator configured to generate two or moresubcarrier frequencies, wherein the waveform generator comprises afrequency synthesizer configured to provide a number of outputfrequencies from a frequency reference to generate the two or moresubcarrier frequencies; a modulator configured to adjust a frequency ofan incident signal at the antenna, the incident signal having a carrierfrequency, the modulator configured to adjust the frequency of theincident signal using at least one of the one or more subcarrierfrequencies to provide a backscattered signal; and an analog switchcoupled between the waveform generator and the modulator and configuredto switch among the two or more subcarrier frequencies.